It has become increasingly apparent over the past decade that the gradual accumulation of polyethylene wear debris in the tissues surrounding artificial joints is a primary cause of long-term loosening failure. The adverse response of the tissues, particularly to the very fine, submicron particles, includes osteolytic destruction of the bone-implant interface. As the techniques for fixation of these prostheses improve, their use is being extended to ever younger patients, and the need to improve the long- term wear resistance of the polyethylene component becomes more acute. The proposed study will investigate the fundamental relationships between the physical properties of polyethylene acetabular cups and their wear resistance, using a hip joint simulator that has been shown to reproduce the lubrication and wear conditions occurring with prosthetic hips in vivo. The study will document the key physical properties of acetabular cups fabricated using conventional processing, and using four modified processes that were developed with the intention of minimizing wear. Physical properties such as crystallinity, density and degree of crosslinking will be correlated with the resultant wear, both for as- fabricated cups and cups subjected to an accelerated ageing environment, intended to model degradation over 5 to 10 years in the body. Wear testing will include ten million wear cycles, and the amount and type of wear will be compared among the conventional and modified acetabular cups, and related to their specific physical properties to provide guidelines for future improvements in wear resistance. The relationship between basic physical properties and the wear performance of the components will be determined for conventional UHMW polyethylene and for four modified polyethylenes that were developed by private industry with the goal of improving the long-term wear performance. Components made using two of these modified processes are already in clinical use. Thus, establishing the relative wear resistance of the conventional and modified materials will provide data of immediate clinical significance, as well as guidelines for future, more effective modifications.